An oxide thin film has conventionally been obtained by dry film formation, such as sputtering or vacuum evaporation, or wet film formation, such as a sol-gel process. However, film formation in dry process requires very expensive equipment. Besides, since different elements have different vapor pressures, it is difficult to produce stoichiometrically excellent films in a stable manner; the resulting films have deteriorated crystal properties; and the productivity is low for the cost. These disadvantages have prevented dry film formation processes from being put to practical use in this field.
On the other hand, a sol-gel method using an organometallic compound is advantageous in terms of low initial cost, precise controllability of chemical composition, uniformity on molecular level, reduction of operation temperature, ease of obtaining large-area films, and the like, and has therefore been given due study in various fields. In general, a sol-gel method is a method for synthesizing an oxide thin film through hydrolysis and dehydrating condensation. It comprises coating a substrate with a solution in which a precursor is hydrolyzed with water, followed by heat treatment.
For example, JP-A-4-19911 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a method for forming a ferroelectric thin film comprising lead titanate (PT), lead zirconate titanate (PZT), third component-doped PZT or lanthanum-doped lead zirconate titanate (PLZT) by using a solution prepared by dissolving an organometallic compound containing lead, lanthanum, titanium or zirconium in an organic solvent and adding thereto a stabilizer and a prescribed amount of water. However, because the process involves hydrolysis using 0.1 to 1.5 mol of water per 1 mol of a metallic atom, the resulting thin film shows grain growth and has an increased leakage current. Further, a polycrystalline oxide thin film obtained by the process is sometimes composed of crystal grains of several microns in diameter depending on the conditions of preparation. Observation of a cross section of such a thin film under a transmission electron microscope (TEM) revealed fine boundary voids among crystal grains in both the surface and the inside thereof as shown in FIG. 1. It seems that a leakage current increases due to electric conduction through these pinholes or boundaries.
JP-B-62-27482 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses a method comprising coating a glass substrate with a solution containing an organometallic compound, drying the coating film in air at ambient temperature for 30 minutes and then in a thermostat set at 110.degree. C. for 30 minutes to complete hydrolysis reaction, and baking the coating layer in an electric furnace at a temperature of 550 to 800.degree. C. while forcibly introducing steam into the furnace. Although hydrolysis does not take place before coating, the resulting thin film still suffers from an increase of leakage current due to the forced introduction of steam into the electric furnace after coating.
Many of these conventionally proposed oxide thin films are utilizable as a ferroelectric thin film. A ferroelectric thin film comprising these oxides exhibits many excellent characteristics, such as ferroelectric properties, piezoelectric properties, pyroelectric properties, electrooptic effects, and the like, and has therefore been used in various fields of electronics. In recent years, application to the memory cells of dynamic RAM (DRAM) has been attracting attention with the rapid advancement of integration, and there has been a demand for an improved process for preparing an oxide thin film.